In commercial jet aircraft applications, a key structural requirement for lower wing and fuselage applications is a high level of damage tolerance as measured by fatigue crack growth (FCG), and fracture toughness. Current generation materials are taken from the Al—Cu 2XXX family, typically of the 2X24 type. These alloys are usually used in a T3X temper and inherently have moderate strength with high fracture toughness and good FCG resistance. Typically, when the 2X24 alloys are artificially aged to a T8 temper, where strength is increased, there is a degradation in toughness and/or FCG performance.
Damage tolerance is a combination of fracture toughness and FCG resistance. As strength increases there is a concurrent decrease in fracture toughness, and maintaining high toughness with increased strength is a desirable attribute of any new alloy product. FCG performance is often measured using two common loading configurations: 1) constant amplitude (CA), and 2) under spectrum or variable loading. The latter is intended to better represent the loading expected in service. Details on flight simulated loading FCG tests are described in J. Schijve, “The significance of flight-simulation fatigue tests”, Delft University Report (LR-466), June 1985. Constant amplitude FCG tests are run using a stress range defined by the R ratio, i.e. minimum/ maximum stress. Crack growth rates are measured as a function of a stress intensity range (? K). Under spectrum loading, crack growth is again measured, but this time is reported over a number of “flights.” Loading is such that it simulates typical takeoff, in flight, and landing loads for each flight, and this is repeated to represent typical lifetime loadings seen for a given part of the aircraft structure. The spectrum FCG tests are a more representative measure of an alloy's performance as they simulate actual aircraft operation. There are a number of generic spectrum loading configurations and also aircraft-specific spectrum which are dependent on aircraft design philosophy and also aircraft size. Smaller, single aisle aircraft are expected to have a higher number of takeoff/landing cycles than large, wide-bodied aircraft that make fewer but longer flights.
Under spectrum loading, an increase in yield strength will often reduce the amount of plasticity-induced crack closure (which retards crack propagation) and will typically result in lower lives. An example has been the performance of a recently developed High Damage Tolerant alloy (designated herein as 2X24HDT) which exhibits a superior spectrum life performance in the lower yield strength T351 temper versus the higher strength T39 temper. Aircraft designers would ideally like to have alloys that possess higher static properties (tensile strength) with the same or higher level of damage tolerance as that seen in the 2X24-T3 temper products.
U.S. Pat. No. 5,652,063 discloses an aluminum alloy composition having Al—Cu—Mg—Ag, in which the Cu—Mg ratio is in the range of about 5-9, with silicon and iron levels up to about 0.1 wt % each. The composition of the '063 patent provides adequate strength, but unexceptional fracture toughness and resistance to fatigue crack growth.
U.S. Pat. No. 5,376,192 also discloses an Al—Cu—Mg—Ag aluminum alloy, having a Cu—Mg ratio of between about 2.3-25, and much higher levels of Fe and Si, on the order of up to about 0.3 and 0.25, respectively.
There remains a need for alloy compositions having adequate strength in combination with enhanced damage tolerance, including fracture toughness and improved resistance to fatigue crack growth, especially under spectrum loading.